In an oxygen cutting operation on a thick part, the heating flames must perform four essential functions, namely:
effectively heat the edge of the slab up to the cut-initiating temperature,
supply on the surface of the part the heat required for maintaining the cutting procedure,
ensure that the cutting jet has satisfactory flow properties,
heat as far into the kerf as possible in order to facilitate the flow of slag.
The cutting oxygen jet must be at a sufficient flow rate to ensure the combustion of the iron and, above all, very "rigid" in order to avoid the drag effects and effectively evacuate the slag, i.e. to convert all the potential pressure energy into flow speed kinetic energy.
At the present time, the most commonly used technology in the design of cutting nozzles for great thicknesses is the double heating ring concentric with a central oxygen jet with an outer ring having a very oxidizing flame for heating the top of the slab to be oxygen cut and a heating ring located between this outer ring and the central cutting jet which has a very carburizing flame for the purpose of obtaining a long feather around the cutting jet which, entrained by the latter, enters the cutting kerf and aids the heating of the middle and bottom of the kerf.
For the oxygen jet, a nozzle termed a Laval nozzle is in most current use for obtaining a high output speed without a bursting of the jet.
However, the use of a very high-speed oxygen jet has its limits, since it may result in insufficient heat in the middle of the kerf which produces, above all when oxygen cutting in the cold state, undermining detrimental to the quality of the cut.
More recently, cutting nozzles have been proposed which have two oxygen jets for the oxygen cutting of current thicknesses, but with off-centre jets.
Further, European patent application No. 0 172 800 discloses an improvement in oxygen cutting methods which employs a supplementary oxygen jet which ensures a re-fusion at the base of the kerf and facilitates the detachment of burrs after cutting.